In recent years, liquid crystal displays have been extensively used as a display device for TVs, personal computers, digital cameras, cellular phones, etc. The liquid crystal displays have no light-emitting function by themselves. Therefore, liquid crystal displays of such a type in which light is irradiated from a backside thereof using a backlight have now come to dominate.
As the backlight type liquid crystal displays, there are known those having a so-called edge light type structure or a so-called direct backlight type structure. With the recent tendency toward reduction in thickness of liquid crystal displays, the edge light type liquid crystal displays have been more frequently employed. The edge light type liquid crystal displays are generally constructed from a reflection sheet, a light guide plate, a light diffusion sheet and a prism sheet which are successively laminated in this order. The flow of light through such edge light type liquid crystal displays is designed such that the light entered from the backlight into the light guide plate is reflected on the reflection sheet and then emitted from the surface of the light guide plate. The light emitted from the light guide plate is entered into the light diffusion sheet, diffused therein and then emitted therefrom. The light emitted from the light diffusion sheet is then entered into the prism sheet disposed next to the light diffusion sheet. In the prism sheet, the light entered thereinto is converged in the normal direction and emitted therefrom toward the liquid crystal layer.
The prism sheet used in the above construction serves for improving an optical efficiency of the backlight and enhancing a brightness thereof. As a transparent base film for the prism sheet, there has been generally used a polyester film in view of a transparency and mechanical properties thereof. In general, an easy-bonding coating layer may be further provided as an intermediate layer between the polyester film as the base material and the prism layer in order to enhance adhesion therebetween. It is known that the easy-bonding coating layer is formed of, for example, a polyester resin, an acrylic resin or a urethane resin (Patent Documents 1 to 3).
The prism layer may be produced, for example, by the following method. That is, an active energy ray-curable coating material is charged into a prism mold, and then a polyester film is placed on the coating material thus charged so as to interpose the coating material between the polyester film and the mold. Next, an active energy ray is irradiated to the active energy ray-curable coating material to cure the resin, and then the mold is removed from the cured resin, thereby obtaining the prism layer formed on the polyester film. In such a method, in order to form an accurate prism pattern on the prism layer, it is required to use a solvent-free type active energy ray-curable coating material. However, the solvent-free type coating material tends to be deteriorated in penetration into an easy-bonding layer laminated on the polyester film and swelling effect therein as compared to a solvent type coating material and, therefore, tends to be insufficient in adhesion to the polyester film. In order to improve the adhesion property, a coating layer comprising a specific urethane resin has been proposed. However, even such a coating layer may still fail to exhibit a sufficient adhesion property to the solvent-free type coating material (Patent Document 4).
Recently, with the considerable reduction in price of liquid crystal displays, there is a demand for considerable reduction in costs required in steps for processing of micro-lens layers or prism layers. As an optimum method for achieving the reduction in costs, there may be mentioned the method using an increased processing speed. However, when merely increasing the processing speed, the amount of active energy rays irradiated tends to be reduced, so that adhesion between the micro-lens layer or prism layer and the polyester film tends to be lowered. In the method of increasing the amount of active energy rays irradiated while increasing the processing speed, there tends to occur increase in costs owing to introduction of lamps or increased amount of electric power consumed, and further there tends to arise such a problem that the polyester film suffers from curling, etc., owing to the heat generation.